1. Field of the Invention
This invention relates to a controller for a power converter which supplies an AC output via a transformer connected in series with an AC system and alters the voltage, current or power of that AC system.
2. Description of the Related Art
An AC system is a gigantic circuit network, and a transmission line can be seen as a distributed constant circuit. As this transmission line becomes longer, its impedance drop becomes greater and its current phase lag also becomes larger. Thus, the power which can be transmitted becomes less.
For this reason, voltage sources are provided in series with existing AC systems. There are power regulation units which vary the size and phase of the voltage impressed on these AC systems from such voltage sources and control the active power and reactive power of the AC systems in order to increase the power transmitted by the AC system according to the load demand.
FIG. 1 is a block diagram of an AC system containing such a power regulation unit.
This is a simplified representation of a general AC system. Transmission line 2 is connected to transmission end 1, and power regulation unit 4 which uses voltage type inverter 3 is connected to transmission line 2.
Here, the voltage and phase at transmission end 1 are E.sub.1 and .theta..sub.1, while the voltage and phase at reception end 5 are E.sub.2 and .theta..sub.2.
To describe the case of there being no power regulation unit 4, the power P transmitted in the AC system is sent from advanced phase in the direction of delayed phase and is expressed by the following equation. Here .delta. is the phase difference angle. EQU P=(E.sub.1 .times.E.sub.2 /X).times.sin .delta. (1)
.delta.=.theta..sub.1 -.theta..sub.2 and X is the impedance of the AC system. PA1 in a controller for a power converter in which a transformer is connected to an AC system and which supplies the AC power of an inverter to the AC system via this transformer and varies the voltage and current, or power, of this AC system, PA1 to provide a controller for a power converter which is equipped with a feedback control device which seeks the voltage drop portion generated by the impedance of the transformer and controls the inverter output so that it compensates this voltage drop portion. PA1 a phase information detector device which detects phase information from the bus voltage and bus current of the AC system; PA1 an impedance drop compensator unit which finds the voltage drop portion generated by the impedance of the transformer based on the current of the AC system, the impedance of the transformer and the phase information detected by the phase information detector device; adders which input the command voltages relating to a first voltage component of the same phase as the bus voltage of the AC system in the voltage supplied via the transformer to the AC system and a second voltage component of a phase which is advanced a specified phase from that phase, subtract the voltage drop portion found by the impedance drop compensator unit from these command voltages, and output the result as inverter voltage commands; and PA1 an inverter control device which restores the inverter voltage command outputted from these adders to the phase of the AC system based on the phase information detected by the phase information detector device, and operationally controls the inverter. PA1 a coordinate conversion unit which outputs a first voltage component of the same phase as the bus voltage by coordinate conversion of the voltage supplied via the transformer to the AC system, and a second voltage component of a phase which is advanced a specified phase from that phase; and PA1 a voltage control unit which finds the deviations between the first and second voltage components outputted from this coordinate conversion unit and these command voltages, and sends voltages which will reduce these deviations to the adders as command voltages. PA1 a signal switching unit which compares the bus current with a pre-determined threshold and, when the bus current is smaller than the threshold, provides the bus voltage to the phase information detector unit and, when the bus current is greater than the threshold, provides the bus current to the phase information detector unit. PA1 a signal switching unit which detects oscillation from the bus current and, depending on the size of this oscillation, provides the bus voltage or the bus current to the phase information detector unit and, when the oscillation does not very from a specified range, provides a signal of the same type as the signal which was used in the past to the phase information detector unit. PA1 a first phase detector which detects phase information for the bus voltage; PA1 a second phase detector which detects phase information for the bus current; PA1 a phase selector unit which outputs a phase selection signal according to the size of the bus current; PA1 filter devices which obtain the difference between the phase of the bus voltage or the phase of the bus current and at least the phase information which is provided to the impedance drop compensator unit or the output of a first-order lag characteristic; and PA1 a phase switching unit which selects the output of the filter devices according to the phase selection signal outputted from the phase selector unit. PA1 a second voltage component arithmetic unit which computes the command voltages for a first voltage component of the same phase for the bus voltage of the AC system and for a second voltage component of a phase specifically phase-advanced from the phase of the first voltage component based on the bus voltage, and applies these command voltages to a voltage control unit. PA1 a voltage command arithmetic unit which computes the first and second voltage components in the voltage supplied via the transformer to the AC system based on the phase information for the bus current and the bus voltage and the impedance command for the transformer, and applies these first and second voltage components to the voltage control unit.
In such as case as this where there is no power regulation unit 4, when a large active power and lag reactive power are required at reception end 5, the voltage drop in transmnission line 2 becomes large. In Equation (1) above, as voltage E.sub.2 at reception end 5 becomes lower, phase .theta..sub.2 becomes more delayed and phase difference angle .delta. becomes larger in order to maintain the transmitted power.
However, to supply stable power, phase difference angle .delta. must be within 90.degree.. Moreover, in practice, there is a requirement to operate by giving some scope to phase difference angle 90.degree., and an that sense there are limits to power transmission.
Also, there are cases when a load exists on the transmission end 1 side and there are generation facilities on the reception end 5 side also, and when transmission of power is carried out by adjusting the power sent in transmission line 2 connecting transmission end 1 and reception end 5. In this case, operation must be performed while balancing by adjusting the voltages and phases of the generation facilities at each of transmission end 1 and reception end 5, and it is extremely difficult accurately to deliver the target power.
However, in power regulation unit 4, the voltage of DC capacitor 7, which is controlled by DC source 6, is converted to AC by inverter 3. This AC is impressed on transmission line 2 via series transformer e, which is inserted in series into transmission line 2. By this means, the power regulation unit has the functions of performing receipt from and delivery to the AC system of its active power and reactive power, and adjusting the active power and reactive power of the AC system.
The following is a concrete explanation of this. On a rotatory coordinate system in which the coordinate of the phase as voltage E.sub.2 of reception end 5 is taken as the d axis and the 90.degree.-advanced phase is taken as the q axis, when the conversions of voltage E.sub.2 of reception end S and bus current I.sub.E are taken as the d and q axis voltages E.sub.2d and E.sub.2q and the d and q axis bus currents I.sub.Ed and I.sub.Eq, q axis voltage E.sub.2q becomes zero.
Active power P and reactive power Q of reception end 5 at this time are expressed as follows. EQU P=E.sub.2d .times.I.sub.Ed (2) EQU Q=-E.sub.2d .times.I.sub.Eq (3)
In order to control the voltage of the AC system so that it does not vary too greatly, from the above Equations (2) and (3) it may be said that active voltage P is proportional to d axis current I.sub.Ed and reactive voltage Q is proportional to q axis current I.sub.Eq. In other words, if the bus current is controlled it is possible to control active power P and reactive power Q of the AC system.
Impedance X of the AC system is mainly the inductance component. Therefore, if d axis component VC.sub.d of output voltage VC of power regulation unit 4 is controlled, it is possible to control q axis current I.sub.Eq of the bus current, and also if q axis component VC.sub.q of output voltage VC is controlled, it is possible to control d axis current I.sub.Ed of the bus current.
For such reasons, power detector unit 9 detects active power P and reactive power Q of the power transmitted on transmission line 2 and outputs them respectively to active power control unit 10 and reactive power control unit 11.
Active power control unit 10 and reactive power control unit 11 are composed of proportional-plus-integral controllers. These respectively output command values VC.sub.d * and VC.sub.q * for the d and q axis components of the series compensating voltage VC so that the active power P and reactive power Q detected by power detector unit 9 will approach the command values.
At the same time, phase information detector unit 12 detects the phase information for bus current I.sub.E and outputs this phase information to coordinate inversion unit 13.
Coordinate inversion unit 13 uses the phase information detected by phase Information detector unit 12 and converts command values VC.sub.d * and VC.sub.q * of the d and q axis components of series compensating voltage VC by inverting them to an alternating current which is synchronised with the bus current and converting it to a 3-phase voltage command to be outputted from inverter 3. It then supplies that 3-phase voltage command to gate control unit 14.
This gate control unit 14 ON/OFF controls the gates of the switching elements which compose inverter 3 in keeping with the voltage commands for inverter 3 from coordinate inversion unit 13.
Here, in series transformer B, since it has impedance, there are errors between command values VC.sub.d * and VC.sub.q * of the d and q axis components and actual values VC.sub.d and VC.sub.q of the d and q axis components of series compensating voltage VC. However, control of active power P and reactive power Q of the AC system is made possible by the actions of active power control unit 10 and reactive power control unit 11.
However, in order to control thoroughly active power P and reactive power Q of the AC system, an extremely large capacity power regulation unit is required. Besides large capacity power regulation units being extremely expensive pieces of equipment, with current technology, their manufacture is difficult in practice.
Also, in a power regulation unit connected to an AC system, apart from control of active power P and reactive power Q of the AC system, the desire is to add the function of performing stabilization when the AC system is oscillating.
This stabilization function is possible with a power regulation unit of a smaller capacity than the capacity required to control thoroughly active power P and reactive power Q of the AC system. For this reason, there is a requirement for the function of performing stabilization of the AC system and to control active power P and reactive power Q of the AC system with a power regulation unit of limited capacity.
When performing the above type of control of active power P and reactive power Q with a power regulation unit of smaller capacity than the transmission capacity of the AC system, the scope in which that control is possible is a very small range. Within this possible control range it is extremely difficult to provide commands to the power regulation unit for active power P and reactive power Q of the AC system.
Therefore, this invention has as its object the provision of a controller for a power converter with a limited capacity which can effectively and stably adjust the active power and the reactive power of an AC system.